Make a group of buttons fill the whole width of the parent div

I have a small panel with a width of 360px. At the top of the panel there is a navigation bar consisting of currently 3 buttons (the number should not matter though). I want the buttons to fill out the whole width of the div and take up more or less space depending on the length of the word inside. Currently I only know how to set the width of the buttons to a value, but then if I add a button the values would need to be changed manually.